Structural integrity monitoring system including wireless electromechanical impedance measurement

ABSTRACT

A structural integrity monitoring system includes a piezoelectric sensor that is adapted to be secured to or embedded within an item of interest. A resistive element is placed in series with the piezoelectric sensor. An output from the series combination of the resistive element and the sensor is conditioned and then transmitted wirelessly to a remote location. An interface located at the remote location receives the transmitted signal, determines the content of the signal and provides an output indicative of the structural integrity of the item of interest.

BACKGROUND OF THE INVENTION

[0001] This invention generally relates to a system for monitoring thestructural integrity of an item. More particularly, this inventionrelates to a system for obtaining electro-mechanical impedanceinformation to determine structural integrity of a chosen item.

[0002] Structural health monitoring is important in various industries.One example is the aerospace industry where mechanical flaws or thesigns of upcoming mechanical flaws are important to locate and address.One hindrance to effectively conducting such monitoring on an ongoing orregular basis is the relatively expensive equipment and cumbersomeprocedures that are currently required.

[0003] One advance in this area has been the implementation of leadzirconate titanate (PZT) piezoelectric sensors that provide informationregarding the item's structural mechanical impedance spectra. As knownin the art, measuring the frequency impedance spectra using PZT sensorsrequires the sensor to be secured to or embedded within the item ofinterest. A major difficulty is presented by the need to communicateinformation from the sensor to a diagnostic tool that provides an outputmeaningful to a technician or other professional who is monitoring thestructural condition of the item. Physical connections between thesensor and other equipment have always been required.

[0004] Such physical connections make the use of such sensors of limitedvalue in many circumstances. For example, a gas turbine engine has manymoving parts and many sensors would be required for an effectivemonitoring arrangement. Introducing multiple sensors, however, includedthe need to introduce multiple hardwired connections or other physicalconnections to other devices within the system This is impractical giventhe nature of a turbine engine, for example. Not only is the taskcumbersome or impossible, but it proves to be overly expensive,susceptible to hardwired connection failures and renders the use of suchsensors impractical under many circumstances.

[0005] There is a need for an effective structural integrity monitoringsystem that can utilize the information gathered with piezoelectricsensors. This invention addresses that need and avoids the shortcomingsand drawbacks of the currently proposed or implemented arrangements.

SUMMARY OF THE INVENTION

[0006] In general terms, this invention is a system for monitoring thestructural integrity of an item. A piezoelectric sensor is supported onthe item of interest. A resistive element is coupled in series with thepiezoelectric sensor. A signal conditioner conditions a signal that hasa component that is indicative of a voltage drop across thepiezoelectric sensor. The series connection of the sensor and theresistive element provide the ability to derive such information in areliable manner. A transmitter transmits the conditioned signal to aremote location, preferably using wireless communication. At the remotelocation, an interface receives the transmitted signal and provides anindication of the structural integrity of the item based upon theinformation within the transmitted signal.

[0007] In one example, the piezoelectric sensor is a PZT sensor that isbonded to the item of interest. A sine wave generator preferablyprovides the voltage that is applied across the series connection of theresistive element and the piezoelectric sensor. The transmitter andinterface device preferably communicate utilizing radio frequencysignals.

[0008] The various features and advantages of this invention will becomeapparent to those skilled in the art from the following detaileddescription of the currently preferred embodiment. The drawings thataccompany the detailed description can be briefly described as follows.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1 schematically illustrates a system designed according tothis invention.

[0010]FIG. 2 is a graphical representation of an example sine sweepelectric signal utilized with the embodiment of FIG. 1.

[0011]FIG. 3A is a graphical illustration of an impedance component of asignal associated with the inventive system.

[0012]FIG. 3B graphically illustrates a differentiation of the signalillustrated in FIG. 3A.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0013] A structural integrity monitoring system 20 provides informationregarding the structural integrity or composition of an item 22 ofinterest. The item 22 may be a component in any one of a variety ofsituations. Examples include aircraft components.

[0014] A piezoelectric sensor 24 preferably is supported on the item 22by being secured to the item or at least partially embedded within theitem. In one example, the piezoelectric sensor 24 is bonded to the item22 in a conventional manner. The preferred embodiment includes a leadzirconate titanate (PZT) piezoelectric sensor.

[0015] A resistive element 26 is coupled in series with the sensor 24.The resistive element 26 preferably has no inductance. A signalgenerator 28 applies a voltage across the series combination of thesensor 24 and the resistive element 26. In one example, the signalgenerator 28 is a sine sweep generator.

[0016] With this invention, sensor 24 acts as both an actuator and asensor that simultaneously induces and transduces vibrations within item22. As known in current practice, an applied electric voltage causeschanges in volume of the actuator 24, which then induces vibrations inthe item 22. Because the sensor 24 is embedded in the item 22, thesevolume changes are a unique representation of the vibrational frequencyresponse of item 22 to the excitation applied by actuator 24. Thesechanges in sensor 24 volume lead to corresponding changes in the currentpassing through sensor 24. As a result, the electric current throughsensor 24 and the voltage drop across the resistive element 26 aremodulated by the vibration of item 22 and are representations of thevibration spectrum of item 22 as driven by the sine sweep signalgenerator 28.

[0017] A signal conditioner 29 obtains a signal at the coupling betweenthe resistive element 26 and the sensor 24. The series combination ofthe sensor 24 and the resistive element 26 effectively operates as avoltage divider of the signal provided by the generator 28. The signalconditioner 29 obtains, therefore, a signal having a component that isindicative of a voltage drop across the sensor 24. In one example, thesignal conditioner 29 is a band pass filter tuned to a selectedfrequency interval. The signal conditioner 29 preferably eliminatesnoise that would interfere with the desired components of the signalobtained from the series combination of the sensor 24 and the resistiveelement 26. Examples of such noise include low frequency noise generatedby environmental vibration of the item 22 and high frequency EMI noise.

[0018] The output from the signal conditioner 29 preferably modulates ahigh frequency carrier signal at a selected frequency. In one example,the carrier signal frequency is 900 MHz. A transmitter 30 wirelesslybroadcasts the signal to an interface 40 located remotely from thesensor 24.

[0019] The interface 40 preferably includes a receiver portion 42 thatreceives and demodulates the transmitted signal from the transmitter 30.A signal processing portion 44 and a computing portion 46 process thereceived signal and provide information regarding the structuralcondition of the item 22.

[0020] The signal processing preferably includes temperaturecompensation. There are known algorithms to compensate for the effect oftemperature on the accuracy of measurements obtained by the inventivesystem. Given this description, those skilled in the art will be able tochoose a suitable temperature compensation strategy.

[0021] In one example, the signal generator 28 is a sine sweepgenerator. FIG. 2 contains a graphical plot 50 illustrating at 52 thevariation in the sine wave signal from a lower frequency ω_(l) to anupper frequency ω_(u). The sinusoidal signal from the generator 28 canbe represented by the following equation:

V _(i)(t)=A·sin(ω(t)·t)  (E1)

[0022] The output signal obtained by the signal conditioner 29 can berepresented by the following equations: $\begin{matrix}\left. {{V_{o}(t)} = {{{V_{i}(t)} \cdot \frac{Z_{es}\left( {\omega (t)} \right)}{{Z_{es}\left( {\omega (t)} \right)} + R}} = {{A^{\prime}\left( {\omega (t)} \right)} \cdot {\sin \left( {\omega (t)} \right)} \cdot t}}} \right) & ({E2}) \\{{{A^{\prime}(\omega)} = \frac{A \cdot {Z_{es}(\omega)}}{{Z_{es}(\omega)} + R}};} & ({E3})\end{matrix}$

[0023] where R is the DC resistance of the resistive element 26. Cracksor other types of damage in the item 22 induce corresponding changes inthe item's mechanical impedance. Such changes are reflected in changesin the electromechanical impedances Z_(es)(ω) of the sensor 24 in theitem 22.

[0024] The interval between the lower frequency ω_(l) and the upperfrequency ω_(u) preferably is chosen to contain several mechanicalresonances of the item 22. At one of the resonant frequencies, theamplitude of the output signal that is received by the signalconditioner 29 is a local maximum. Changes to the structural integrityin the item 22 cause changes in the resonant frequencies. Additionallyor alternatively, significant qualitative changes in the spectrum A′(ω),such as peak splitting (e.g., as can result from damage-induced loss ofsymmetry of the item 22) or the disappearance of peaks, may result.

[0025] After the signal conditioner 29 has filtered the output signalcontaining a component indicating a voltage drop across the sensor 24,the conditioned signal is wirelessly broadcast by the transmitter 30.The signal broadcast by the transmitter can be described by thefollowing equation:

V _(m)(t)=V _(o)(t)·sin(ω_(o) t)  (E4)

[0026] Once received by the interface 40, the receiver 42 preferablydemodulates the signal back into the original output signal V_(o)(t)described by the above equation E2. The signal processing portion 44preferably then transforms the output signal into a signal thatrepresents the envelope of the output of the signal conditioner 29. Thisenvelope signal also provides information regarding theelectromechanical impedance spectrum of the sensor 24 and the item 22.

[0027] The reconstructed impedance spectrum preferably is then processedusing an analog to digital channel on a computer 46 or microprocessorthat is appropriately programmed for data acquisition regarding thestructural condition of the item 22. Given this description, thoseskilled in the art will be able to appropriately program a computer toutilize the information from the obtained signal to provide an outputregarding the structural condition of the item. The output of theinterface 40 may be visual as illustrated in the component 46 or may bestored to computer memory, or both, depending on the needs of a givensituation.

[0028] Another feature of this invention is synchronizing the signal ofthe signal generator 28 with data acquisition at the interface 40. Inthe currently preferred embodiment, the recovered impedance signatureprovided by the processing portion 44 is differentiated. In one example,an R-C circuit (not illustrated) differentiates the signal before it isprovided to an analog to digital channel on a computer. Thedifferentiation provides sharp pulses at times corresponding to thetimes t_(o), t₁, and t₂ of FIG. 2.

[0029] As seen in FIG. 3A, the impedance signature 60 is a continuousand slow changing function of the frequency ω. Between the various timest_(i), the differentiated signal 62 has a relatively small value withinthe intervals between those discrete times. At each of the discretetimes t_(i), however, there is a step change in the sine sweep frequencyω from the lower value to the upper value and a corresponding stepchange in the amplitude of the impedance signature signal 60. Therefore,the result of differentiating the signal 60 at the discreet times t_(i)provides the signal with a relatively large value at each of theseinstances. The arrangement of the differentiation portion of theinterface 40 preferably is chosen so that the differentiated signals atthe times t_(i) are significantly larger than at any of the timesbetween those instances. In one example, the R-C circuit is chosen toprovide this result.

[0030] Within the selected sweep frequency band, the pulse of thedifferentiated signal uniquely identifies the start of each sweepperiod. An example pulse is shown at 64. Each pulse, therefore, providessynchronization information to the interface 40 regarding the operationof the signal generator 28. The interface 40 preferably is programmed touse such pulse information to synchronize data acquisition of thereceived signal with the operation of the signal generator 28.

[0031] This invention represents a significant improvement in thestructural integrity monitoring art. The inventive system provides theability to wirelessly transmit signals that include a representation ofthe impedance spectra that indicates the structural integrity of theitem 22.

[0032] The preceding description is exemplary rather than limiting innature. Variations and modifications to the disclosed embodiment maybecome apparent to those skilled in the art that do not necessarilydepart from the purview and spirit of this invention. The scope of legalprotection given to this invention can only be determined by studyingthe following claims.

We claim:
 1. A system for determining a structural condition of an item,comprising: a piezoelectric sensor that is adapted to be supported onthe item; a resistive element coupled in series with the piezoelectricsensor; a signal conditioner that conditions a signal including anindication of a voltage drop across the sensor; a transmitter thattransmits the processed signal; and a remotely located interface thatreceives the transmitted signal and provides an output indicative of animpedance of the processed signal and the structural condition of theitem.
 2. The system of claim 1, wherein the resistive element has noinductance.
 3. The system of claim 1, wherein the signal conditionerincludes a bandpass filter.
 4. The system of claim 3, wherein thebandpass filter removes signal components below approximately 50 KHz andabove approximately 200 KHz.
 5. The system of claim 1, wherein thetransmitter and the remotely located interface communicate usingwireless signal transmission.
 6. The system of claim 5, wherein thetransmitter and the remotely located interface utilize radio frequencysignal communication.
 7. The system of claim 1, wherein the interfaceincludes a portion that determines a mechanical impedance value of theprocessed signal and determines an indication of the structuralcondition from the impedance value.
 8. The system of claim 1, includinga varying voltage generator that applies a voltage across the resistiveelement and the sensor.
 9. The system of claim 8, wherein the voltagegenerator is a sine sweep generator.
 10. The system of claim 1,including a differentiating portion that differentiates an impedancevalue of the transmitted signal over time and wherein the interfacesynchronizes data acquisition from the transmitted signal with at leastone selected value of the voltage generator.
 11. The system of claim 10,wherein the differentiating portion includes an RC circuit.
 12. A methodof determining a structural condition of an item, comprising the stepsof: (a) attaching a piezoelectric sensor to the item; (b) coupling aresistive element in series with the sensor; (c) transmitting a signalthat includes an indication of a voltage drop across the sensor to aprocessor located remotely from the sensor; and (d) determining astructural condition of the item from the transmitted signal.
 13. Themethod of claim 12, including embedding the sensor within a portion ofthe item, using the sensor as an actuator to induce vibrations in theitem and simultaneously monitoring vibration in the item using thesensor.
 14. The method of claim 12, wherein step (C) includes usingwireless communication.
 15. The method of claim 14, wherein step (C)includes using radio frequency communication.
 16. The method of claim12, wherein step (D) includes determining an impedance value of thetransmitted signal using the indication of the voltage drop and usingthe impedance value to determine the structural condition.
 17. Themethod of claim 12, including applying a voltage with a varyingfrequency across the resistive element and the sensor anddifferentiating the impedance value of the transmitted signal over timeto thereby determine synchronization indicators and using the indicatorsto synchronize data acquisition from the transmitted signal with thevarying voltage.
 18. The method of claim 17, including using a sinesweep generator.
 19. The method of claim 12, wherein step (C) includesconditioning the signal to remove selected frequency components prior totransmitting the signal.
 20. The method of claim 12, wherein the voltagedrop of step (C) is frequency dependent.